Each spring, millions of monarch butterflies leave their overwintering sites in the Sierra Madre mountains of central Mexico and begin their annual migration north across the United States. The exodus and return of the iconic orange and black butterfly is one of the grandest spectacles of the natural world. But that sight is becoming increasingly rare as the monarch’s population has shrunk by nearly 90% in the past two decades, according to federal scientists. The monarch faces many threats, including the loss of milkweed and other flowering plants across its range, degradation and loss of ove…